State of the art SerDes transceivers signaling at rates on the order of 3.125 Gbps through typical FR4 back-plane channels utilize a simple 2-tap FIR (finite impulse response) transmit filter to equalize frequency dependent channel loss. One of the tap values is adjustable with respect to the other. This scheme is often referred to as pre-emphasis. Since there is only one tap value to adjust, it is left to the user to choose the right value for a given channel.
For data rates above 5 Gbps on FR4 back-planes, however, a simple 2-tap FIR filter is not sufficient to equalize for high frequency channel loss. Proposed solutions include an N-tap FIR transmit filter plus a fixed high boost filter at the receiver. For N>3 it becomes difficult for the user to select appropriate values of each filter coefficient for every channel used by the transceiver, hence, adaptive equalization is utilized. Since the N-tap FIR filter is implemented in the transmitter, the receiver must relay information on tap value updates back to the transmitter in order for the FIR filter to adapt to the channel.
Various techniques have been developed to solve the problem of relaying tap value updates from the receiver back to the transmitter.
Some solutions require insertion of additional data on a back channel to indicate tap update information, which results in a small increase in the channel data rate.
Other solutions require a separate physical channel to relay the tap update information. Apart from being an awkward solution, this solution rules out the use of the adaptive scheme on older systems where the additional physical back channel is not present.
Still other solutions propose that a supervisor chip read the information on tap value updates from the receiver and send it to the transmitter (both through an MDIO (management data input/output) interface, for example). Again, this solution unnecessarily involves processor overhead.